1. Field of the Invention
The present invention relates to an ink jet apparatus and, more particularly, to an ink jet apparatus that operates by the deformation of piezoelectric ceramics.
2. Description of the Related Art
Known ink jet printer heads operate on the so-called drop-on-demand method utilizing a piezoelectric ceramic arrangement. This type of ink jet printer head involves having the piezoelectric ceramic arrangement deformed to vary the volumes of ink chambers formed therein. When the volume of a given ink chamber is reduced, the ink inside that ink chamber is jetted out through a nozzle in the form of droplets; when the volume of an ink chamber is expanded, additional ink is introduced into that ink chamber through a separately provided ink conduit. A large number of such ink chambers are positioned close to one another. The nozzles coupled to the ink chambers jet out ink droplets selectively according to appropriate print data. The process forms characters or images onto paper or other suitable medium positioned opposite to the nozzles.
Typical ink jet apparatuses of this kind are disclosed illustratively in U.S. Pat. Nos. 4,879,568, 4,887,100 and 5,016,028. FIGS. 15, 16, 17 and 18 outline these apparatuses. A typical constitution of this kind of ink jet apparatus is described referring to FIG. 15 which is a cross-sectional view of the prior art apparatus. In FIG. 15, a piezoelectric ceramic plate I comprises a plurality of grooves 15 and side walls 11 that separate the grooves 15. The ceramic plate 1 is polarized in the direction of arrow 4. A cover plate 2 is made of ceramic or plastic resin. The piezoelectric ceramic plate 1 and the cover plate 2 are bonded together with a junction layer 3 interposed therebetween. The junction layer 3 is composed of epoxy resin adhesive or the like. In this structure, the grooves 15 form a plurality of ink chambers 12 spaced apart crosswise. Each ink chamber 12 has a rectangular cross section and is long and narrow in shape. Each side wall 11 extends along the entire length of the ink chamber. Both sides of each wall 11 from the wall top near the junction layer 3 to the approximate middle of the wall are furnished with metal electrodes 13 that apply driving electric fields. All ink chambers 12 are filled with ink during operation.
The operation of the above ink jet apparatus is described referring to FIG. 16, which is another cross-sectional view of the prior art apparatus. In operation, an ink chamber 12b is illustratively selected according to the print data supplied. Then metal electrodes 13e and 13f rapidly apply a positive driving voltage, while metal electrodes 13d and 13g are connected to ground. This causes a driving electric field to develop on a side wall 11b in the direction of arrow 14b and another driving electric field to develop on a side wall 11c in the direction of arrow 14c. Because the directions of the driving fields 14b and 14c are each perpendicular to the direction of polarization 4, the side walls 11b and 11c are deformed rapidly into the ink chamber 12b due to the so-called piezoelectric thickness slip effect. The deformation reduces the volume of the ink chamber 12b and rapidly raises the ink pressure therein, generating pressure waves that cause ink droplets to jet out of a nozzle (FIG. 17) connected to the ink chamber 12b. When the driving voltage is gradually deactivated, the side walls 11b and 11c return to their initial positions. This gradually reduces the ink pressure inside the ink chamber 12b, introducing ink thereinto through an ink supply port 21 and a manifold 22 (FIG. 17).
Only the basic operation of the prior art apparatus is described above. When incorporated in specific printers, the ink jet apparatus may work in a somewhat different manner. That is, the driving voltage may be applied initially in a direction that will increase the volume of the ink chamber 12b to fill it with ink, followed by the deformation of the side walls to jet the ink out.
The construction and manufacture of the prior art apparatus is described illustratively with reference to FIG. 17, which is an exploded view in partial section. The piezoelectric ceramic plate 1 is first polarized and then cut by a thin disc-shaped diamond blade tool or the like to form the grooves 15 arranged in parallel. The grooves 15 form the ink chambers 12 as mentioned above. While the parallel grooves 15 have substantially the same depth over the entire area of the piezoelectric ceramic plate 1, the grooves become somewhat shallower as they approach a plate edge 17. Near the edge 17, the grooves 15 are replaced by shallow grooves 18 also arranged in parallel. The inner surfaces of the parallel grooves 15 and 18 are furnished with the metal electrodes 13. The electrodes are deposited on the wall surfaces by sputtering or by other suitable processes. While only the upper half of the side walls of the grooves 15 is equipped with the metal electrodes 13, the entire side walls and the bottoms of the shallow parallel grooves 18 are covered with the metal electrodes. Furthermore, ink supply ports 21 and manifolds 22 are ground or cut through the cover plate 2 made of ceramic or plastic resin.
The groove-cut side of the piezoelectric ceramic plate 1 and the manifold-formed side of the cover plate 2 are bonded together, preferably using epoxy resin adhesive or the like. The two plates are bonded so that the ink chambers 12 of the above-mentioned shape will be formed therebetween. The outer edge 16 of the piezoelectric ceramic plate 1 and the outer edge of the cover plate 2 are bonded to a nozzle plate 31. The nozzle plate 31 has nozzles 32 corresponding to the positions of the ink chambers 12. A substrate 41 is bonded to the surface opposite to the groove-cut side of the piezoelectric ceramic plate 1 by epoxy resin adhesive or the like. The substrate 41 has conductive layer patterns 42 corresponding to the positions of the ink chambers 12. The metal electrodes at the bottoms of the shallow parallel grooves 18 are connected to the conductive layer patterns 42 by use of conductors 43 deposited by wire bonding.
The construction of the control section of the prior art apparatus is described with reference to FIG. 18, which is a schematic diagram of the control section. Each of the conductive layer patterns 42 on the substrate 41 is connected individually to an LSI chip 51. Also connected to the LSI chip 51 are a clock line 52, a data line 53, a voltage line 54 and a grounding line 55. Given continuous clock pulses through the clock line 52, as well as data from the data line 53, the LSI chip 51 decides through which nozzles ink droplets are to be jetted out. Based on its decision, the LSI chip 51 selectively applies the voltage V of the voltage line 54 to the conductive layer patterns 42 connected to the metal electrodes that belong to the target ink chambers. The LSI chip 51 also applies a zero voltage of the grounding line 55 to those conductive layer patterns connected to the metal electrodes that do not belong to the target ink chambers.
One disadvantage of the above-described prior art ink jet apparatus is a low ink pressure that occurs inside the ink chambers. This is attributable to the fact that the side walls made of piezoelectric ceramics are not deformed appreciably despite high levels of electric energy applied to the metal electrodes. The comparatively limited reductions in the volumes of the ink chambers result in the low ink pressure therein. The available ink pressure is not enough to ensure a sufficiently high velocity and a sufficiently large volume of ink droplets jetted onto paper or like material to form characters and images successfully on the paper positioned opposite to the printer head. If the prior art apparatus is desired to jet ink droplets at velocities and in volumes sufficient for the formation of characters and images, the apparatus is required to handle high driving voltages. To meet this requirement, a complicated large-size driving circuit must be built that puts severe constraints on any costs containing and size reduction efforts.